The Environmental Protection Agency (EPA) requires vehicle manufacturers to install on-board diagnostics (OBD-II systems) for monitoring light-duty automobiles and trucks beginning with model year 1996. OBD-II systems (e.g., microcontrollers and sensors) monitor the vehicle's electrical, mechanical, and emission systems and generate data that are processed by a vehicle's engine control unit (ECU) to detect malfunctions or deterioration in the vehicle's performance. Most ECUs transmit status and diagnostic information over a shared, standardized electronic buss in the vehicle. The buss effectively functions as an on-board computer network with many processors, each of which transmits and receives data. Sensors that monitor the vehicle's engine functions (e.g., the cruise-control module, spark controller, exhaust/gas recirculator) and power train (e.g., its engine, transmission, and braking systems) generate data that pass across the buss. Such data are typically stored in memory in the ECU and include parameters such as vehicle speed, fuel level, engine temperature, and intake manifold pressure. In addition, in response to these data, the ECU generates 5-digit ‘diagnostic trouble codes’ (DTCs) that indicate a specific problem with the vehicle. The presence of a DTC in the memory of a vehicle's ECU can result in illumination of the ‘Malfunction Indicator Light’ (MIL) present on the dashboard of most vehicles. When the MIL is lit a corresponding datum on the ECU is stored with a value of ‘1’, while an unlit MIL has a corresponding datum of ‘0’.
The above-mentioned data are made available through a standardized, serial 16-cavity connector referred to herein as an ‘OBD-II connector’. The OBD-II connector is in electrical communication with the ECU and typically lies underneath the vehicle's dashboard.
the EPA has also recommended that inspection and maintenance (I/M) readiness tests conducted using the OBC-II connector be used to diagnose a vehicle's emissions performance. I/M readiness tests monitor the status of up to 11 emissions control-related subsystems in a vehicle. The ECU monitors first three subsystems—misfire, fuel trim, and comprehensive subsystems—continuously. The remaining eight subsystems—catalyst, evaporative system, oxygen sensor, heated oxygen sensor, exhaust gas recirculation (EGR), air conditioning, secondary air, and heated catalyst subsystems—are run after a predetermined set of conditions are met. Not all subsystems (particularly the air conditioning, secondary air, and heated catalyst subsystems) are necessarily present on all vehicles.
I/M readiness tests generate a ‘flag’ describing their status. The flag can appear as either ‘complete’ (meaning that the test in question has been successfully completed), ‘incomplete’ (meaning that the test has not been successfully completed), or ‘not applicable’ (meaning that the vehicle is not equipped with the subsystem in question).
Current federal regulations for I/M readiness testing are described in 40 CFR Parts 51 and 85, the contents of which are incorporated herein by reference. In general, these regulations require that a vehicle manufactured during or after model year 2001 having an I/M readiness flag of ‘incomplete’ does not ‘pass’ the emissions test. Other vehicles that do not ‘pass’ the test include those manufactured between model years 1996 and 2000 with more than two ‘incomplete’ readiness flags, and those manufactured in model year 2000 with more than one ‘incomplete’ flag. In addition, the regulations require that any vehicle that includes a DTC that lights its MIL does not ‘pass’ the test. A vehicle with a malfunctioning MIL (e.g., a MIL that includes a burnt-out bulb) also does not ‘pass’ the test.
During existing I/M inspections, data from the vehicle's ECU is typically queried using an external engine-diagnostic tool (commonly called a ‘scan tool’) that plugs into the OBD-II connector. The vehicle's engine is turned on and data are transferred from the ECU, through the OBD-II connector, and to the scan tool. The scan tool then displays and analyzes the data to monitor the vehicle. Scan tools are typically only used to diagnose stationary vehicles or vehicles running on a dynamometer.